Simplified extractive distillation process for obtaining aromatics of differing boiling-point ranges

ABSTRACT

AN EXTRACTIVE DISTILLATION PROCESS FOR SIMULTANEOUSLY OBTAINING AROMATICS WITH DIFFERENT BOILING-POINT RANGES FROM A FEED STOCK CONTAINING VARIED AMOUNTS OF NONAROMATICS ALONG WITH THE DESIRED AROMATICS, SAID PROCESS BEING OF THE TYPE WHEREIN THE FEED STOCK IS PASSED THROUGH AT LEAST ONE EXTRACTIVE DISTILLATION COLUMN AND IS CONTACTED WITH A SELECTIVELY ACTIVE EXTRACTION AGENT TO WHICH AROMATICS OTHER THAN THE DESIRED AROMATICS HAVE BEEN ADDED, AND WHEREIN THE MIXTURE OF AROMATICS AND EXTRACTION AGENT IS WITHDRAWN FROM THE BOTTOM OF THE COLUMN WHILE WITHDRAWING THE NON-AROMATICS IN VAPOR FORM FROM THE TOP OF THE COLUMN, THE IMPROVEMENT COMPRISING PASSING THE AROMATICS CONTAINING MASS RECEIVED FROM THE SAID EXTRACTIVE DISTILLATION COLUMN THROUGH A PLURALITY OF IN-SERIES CONNECTED COLUMN SYSTEMS, THE NUMBER OF SAID SYSTEMS CORRESPONDING TO THE NUMBER OF AROMATICS TO BE OBTAINED AND SUBJECTING THE MASS IN EACH OF SAID COLUMN SYSTEMS SUCCESSIVELY TO AN EXTRACTIVE DISTILLATION AND TO A NORMAL DISTILLATION AND RECOVERING FROM EACH SYSTEM THE DESIRED AROMATIC FRACTION.

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' SIMPLIFIED EXTRASTIYE DISTILLATION ,PROCESS FOR OBTAINING AROMATICS OF DIFFERING BOILING-POINT RANGES Filed. Oct. 11, 1967 2 Sheets-Sheet 1 I f r 1 7 r-w 1 -41 H ig ..2' Q

Jan. 12,1971 H ETAL 3 ,554,8 73

' SIMPLIFIED EXTRASTIVE DIS'IILLATION PROCESS FOR OBTAINING I I AROMATI'CS OF DIFFERING BOILING-POINT! RANGES Filed Oct. I1, 1967 1 2 Sheets-Sheet 2 Fig.3

United States Patent 5 Int. Cl. B01d 3/34,- c07 7/00 US. Cl. 203-51 14 Claims ABSTRACT OF THE DISCLOSURE An extractive distillation process for simultaneously obtaining aromatics with different boiling-point ranges from a feed stock containing varied amounts of nonaromatics along with the desired aromatics, said process being of the type wherein the feed stock is passed through at least one extractive distillation column and is contacted with a selectively active extraction agent to which aromatics other than the desired aromatics have been added, and wherein the mixture of aromatics and extraction agent is withdrawn from the bottom of the column while withdrawing the non-aromatics in vapor form from the top of the column, the improvement comprising passing the aromatics containing mass received from the said extractive distillation column through a plurality of in-series connected column systems, the number of said systems corresponding to the number of aromatics to be obtained and subjecting the mass in each of said column systems successively to an extractive distillation and to a normal distillation and recovering from each system the desired aromatic fraction.

CROSS-REFERENCES TO RELATED APPLICATIONS Application Ser. No. 627,788, filed on Apr. 3, 1967, by the same inventors as the present application and patented May 20, 1969 as US. Pat. No. 3,445,537 and relating to Extractive-Distillation Process for Obtaining Aromatics of Differing Boiling-Point Ranges and assigned to the same assignee as the present application.

BACKGROUND OF THE INVENTION The use of an extractive distillation process for obtaining pure aromatics from mixtures containing nonaromatic hydrocarbons together with the aromatics has been disclosed, for instance, in German published application No. 1,206,414. The feed stock in this process is introduced at about the vertical center of the extractive distillation column and is contacted with an extraction agent that flows from top to bottom through the column and is selectively active towards the aromatics. The aromatics are withdrawn in that process together with the extraction agent from the bottoms of the column and are separated by subsequent distillation. The nonaromatics are simultaneously caused to vaporize in the extractive distillation column and are withdrawn in vapor form through the top thereof.

This process, due to the simple arrangement, has many advantages over the liquid-liquid extraction. To name a few: An installation for recovery of the extraction agent from the raflinate is eliminated and, in general, complex mechanical installations are avoided, such as a multiplicity of revolving pumpsand similar apparatus. The viscosity of the extracting agent is substantially reduced through the applied higher temperature, which results in a material improvement of the product exchange between extraction agent and product to be extracted. One obtains, therefore, improved charge potentialities and can get along with smaller amounts of extraction agent at comparable performances. The dimensions of the extractor can also be reduced in most cases. These last two advantages result in cost-saving when the extractive distillation is compared with the liquid-liquid extraction.

The extractive distillation unfortunately has two short comings as against the liquid-liquid extraction. In the first place, there is a comparatively high portion of extraction liquid in the rafiinate phase of the extractive distillation plant. In the second place, the problem of obtaining products with widely difiering boiling-point ranges from the extractive distillation process is difficult to solve.

As stated in the copending case, it was found that aromatics appear along with the desired parafiins in the raflinate phase even when using extraction agents that are free of aromatics. This phenomenon is in particular noted in the raffinate phase of extractive distillation plants employed to separate aromatics from non-aromatics. For instance, working with a propylene carbonate as extraction agent there were found contents of benzene between 10% and 30% by weight in the raflinate phase, depending on working conditions. This high share of aromatics in the raflinate phase (paraffin fraction) amounts to an uneconomical and low yield, particularly where the feed material contains a low amount of aromatics. The extractive distillation method has therefore been used heretofore only for feed products with a content of non-aromatics up to about 20%, since in these cases the mentioned shortcomings were without significance, or otherwise the extractive distillation Was carried out in combination with a liquid-liquid distillation.

The reason for the presence of aromatics in the rafiinate phase (paraflin fraction) was found to be that two liquid layers are formed on the top plates of the extractive distillation column. In other words, there are concentration conditions which result in a system having a mixture gap.

In this copending process the non-aromatics containing mass drawn off from the extraction columns may be subjected to further treatment in separate auxiliary columns where any residues of lower boiling-point aromatics that may still be present are removed from the product, whereupon the bottom product of the auxiliary columns which substantially no longer contains any aromatics may be subjected to another extractive distillation. Preferably this is done by collecting the different masses of non-aromatics products from the several extractive distillation columns and subjecting them to another common extractive distillation. If desired, the final refining may also be effected in a normal distillation step.

SUMMARY OF THE INVENTION It is an object of the present invention to provide for a modification and improvement of the process of the copending application. In a more specific way, it is an object of the present application to dispense with the justdescribed after-treatment of the nonaromatics in a separate column or in several such columns. It is therefore also an object of the present invention to improve the economics of the earlier process in regard to necessary apparatus and operating costs by eliminating the separate column or columns for the after treatment.

The improvement consists in passing the mass to be processed after it comes from the extractive distillation column or columns through a plurality of in-series connected column systems, the number of said systems corresponding to the number of aromatics to be obtained, and subjecting the mass in each of said column systems successively to an extractive distillation and to a normal distillation and recovering from each system the desired aromatic fraction.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims .The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates, in a diagrammatic form, one single column system for use in the process of the invention;

FIG. 2 illustrates a modified form of such column system; and

FIG. 3 is a flow sheet illustration of the extractive distillation process and apparatus of the invention. Secondary parts of apparatus such as evaporators, cooling-apparatus, heat exchangers, pumps and valves are omitted in the flow sheet to give a clear picture of the process.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the invention accordingly differs from that of the mentioned copending application in that further separation of the extraction product mixture enriched by the preceding extractive-distillations, into individual fractions, takes place with the aid of a column system whose column sections not only perform the function of extractive-distillation but also the function of normal distillation. At the top of the extractive-distillation portion composed of the extractive stripper section and the extractive rectifying section there are drawn off a lower boiling aromatic component, such as benzene, and a refined (non-aromatics) product, having a boiling point Within the range of the next higher boiling aromatic component, such as toluene.

This top product of the extractive-distillation is subsequently split, by distilling, into benzene and the nonaromatics of the toluene boiling range.

Those aromatics which have a higher boiling point range than that obtained from the first column system may be similarly obtained from downstream connected column systems.

The concept of the column system used in the method r of the invention is best explained by reference to FIG. 1. The column system shown therein consists of the four column sections 1a, 1b, 1c and 1d respectively. The column sections 1a and 1b both primarily operate in the mode of extractive distillation, section 1a being an extractive stripper section and section 1b being an extractive rectifying section. The column sections 10 and 1d both operate in the mode of normal distillation, section 10 being a distillative stripper section.

The extractive column sections 1a and 1b are interconnected by the product streams 4 and 5 in the same way as in a normal extractive-distillation column, both on the vapor side as well as on the liquid side.

The distillative column sections 10 and 1d are interconnected by the product streams 6 and 7 as in normal distillation, on the vapor side as well as on the liquid side.

The extractive column, consisting of sections 1a and 1b, is connected at its head by the product stream 8 to the distillative column consisting of sections 10 and 1d. This product stream 8, which may be vapor or liquid, is fed to the uppermost plate of the distillative stripper section Id.

In the operation of such a column system, extraction agent is supplied through a pipeline 2 to the uppermost plate of the extractive rectifying section 1b, a feed mixture obtained from the bottom of a preceding column being supplied through a pipeline 3 to the uppermost plate of the extractive stripper section 1a, whilst a reflux is supplied through a pipeline 12 to the uppermost plate of the distillative rectifying section 10.

The extraction product (e.g. aromatic) fraction to be obtained from such system is tapped off through pipeline 9, whilst the refined product (e.g. non-aromatic) fraction, having the boiling range of the extraction product fraction of the next column system, is tapped off through pipeline 10.

The bottom of the extractive stripper section 1a, consisting substantially of extraction agent and residual aromatics is tapped off through pipeline 11. Where appropriate, this bottom product may be supplied to a subsequent column system for further processing.

The column system shown in FIG. 1 may take various forms; the individual column sections may, for instance, be arranged as illustrated in FIG. 2, in which the reference numerals have the same significance as in FIG. 1. The embodiment illustrated in FIG. 2 differs from that of FIG. 1 in that the extractive rectifying section 1b and the distillative rectifying section 10 are both constructed as parts of a single column unit also including the extractive stripper section 1a. Other form's are also possible.

As regards the procedure (a) referred to hereinbefore, this can be accomplished by allowing to remain in the extraction agent those extraction product-like substances (e.g. aromatics) originating from the feedstock which are not to be extracted or are to be only partially extracted. It is of course also possible to add to the extraction agent other extratcion product-like substances (aromatics) not to be extracted and not present in the feedstock. For example, it is possible to add toluene to the extraction agent if benzene and xylene are to be obtained by extractive-distillation of a feedstock containing these aromatics mixed with parafiins. It is also possible to add to the extraction agent xylene or C aromatics which are not to be extracted from such a feedstock in a plant in which only benzene is to be extracted. The amount of extraction product-like substances (e.g. aromatics) to be added to the extraction agent may be up to 20% by volume referred to the extraction agent.The amount of such an additive also depends on the pressure at which extractivedistillation is performed. This is subject to the rule that under condtions of elevated pressure (positive pressure) the amount of additive may be correspondingly reduced.

Increasing the working pressure of the first extractivedistillation column results in an increase of the solubility of the refined product (e.g. non-aromatics) in the extraction agent. This procedure leads to some deterioration in the selectivity of the extraction agent. The resultant disadvantages may however be compensated by freeing the extraction agent/refined product mixture, in a downstream connected extractive-distillation system operated at lower pressure (atmospheric or subatmospheric pressure), of refined product fractions which can be removed only with great difiiculty. The method of operation according to the invention is however not confined to the use of positive pressure in the first extractive-distillation column.

According to a further feature of the invention, the extractive rectifying section 1b may be omitted if a yield of the next higher boiling extraction product (e.g. aromatic) fraction is not required. Moreover, it is possible to dispense with the distillative stripper section 1d if a yield and purification of the heads products of this column system (aromatic) are not required. Finally, an additional column section may be mounted upon the head of the extractive rectifying section 1b, the head of the aforementioned column section being fed with ex traction product-like substances (e.g. aromatics) or a fraction containing such substances and 'which serves for the additional extraction agent recovery.

The method of the invention may further include the step of returning to the extractive-distillation system a bottom product obtained from the last extractive stripper section and which consists of extraction agent feed of the required extraction products e.g. aromatics. The sensible heat of this bottom product as well as of the bottom product of the extractive-distillation columns operated at higher pressures may then be employed for heating the extractive-distillation columns operated at lower pressures and/ or the distillative stripper sections.

At the head of the extractive stripper secion certain losses of extraction agent may occur. However, these losses may be easily avoided by arranging for a small part of the high boiling extraction products-like substances (e.g. aromatics) which are not to be extracted to be introduced as washing medium into the extraction column above the extraction agent feed, instead of adding these substances directly to the extraction agent. These substances 'will then wash the extraction agent from the vaporized refined ,product (e.g. non-aromatics.)

It should be understood that the method according to the invention is not confined to the use of a particular extraction agent, the criterion for suitability being an adequate selectivity for separating the extraction products from the refined products. The following substances are suitable extraction agents for separating aromatics from paraffins: propylene carbonate, N-oxyalkyl derivatives for 1,3-propane diamine and N-substituted morpholine. This enumeration does not, however, represent any limitation to the aforementioned compounds. Many other compounds may also be employed as extraction agents. The method of the invention can be employed for the separation of any desired kinds of substances with any desired extraction agents if the attainably purity of the heads product of extractive-distillation is limited by a miscibility gap.

Referring now to FIG. 3, this illustrates in block diagram-flowsheet form the principal plant components used, typically, to carry out the method of the invention for separating aromatics from paraflins: in this figure, all auxiliary devices which are of a lower order of significance for the method, such as heaters, coolers, heat exchangers, pumps, valves and the like, have 'been omitted in the interest of clarity. The process will be described in relation to its use for refining a feedback consisting of a hydrocarbon mixture having the following composition by weight (in. kg.):

For the purpose of this description, a feed rate of 1000 kg. per hour will be assumed and the quantities of materials are expressed on this hourly basis.

When using a feedstock of the above-mentioned composition, the weight ratio of extraction agent to feedstock should be equal to 3, that is 3000 kg. of extraction agent are employed. The extraction agent consists of approximately 2880 kg. propylene carbonate and approximately 120 kg. toluene. Approximately 80% of this quantity of toluene is fed through a line 41 to the head of a first extractive-distillation column 14 and the remaining 20% is fed through a line 42 to the head of a second extractivedistillation column 19. The toluene employed in the extraction medium may at least partly originate in the feedstock being extracted therefrom in the process as will be described and fed by recycle line 32 to the pipelines 41 and 42 from the toluene Output line 31.

The feedstock is introduced through a line 13 to a middle plate of the first extractive-distillation column 14 which operates at a pressure of approximately 3.5 atm.

C aromatics The appropriate amount of the propylene carbonate constituent of the extraction agent is fed through a line 15 to one of the upper plates of the column 14. A large proportion of the non-aromatics in the feed mixture escapes in vapour form through a line 16 at the head of the column and is fed into a manifold line 17. The bottom product of the column 14 which contains the aromatics and the extraction agent in addition to the residue of nonaromatics passes through a line 18 to constitute the feed for the second extractive-distillation column 19 which operates at normal (atmospheric) pressure. The feed in this case is also introduced to one of the middle plates of the column 19. Propylene carbonate is fed through the line 20 to one of the upper plates of the column 19. In this column 19, the non-aromatics escape in vapour form through a line 21 at the head of the column and, after condensation, are also fed into the manifold line 17.

The bottoms product of the column 19 is conveyed through a line 22 into a first column system 23 of the form illustrated in FIG. 2 sections 23a, 23b and 230 corresponding with sections 1a, 1b and 1c of FIG. 2 and being combined in a single column. The side column 23d of the system 23 is a distillative stripper section corresponding with section 1d in FIG. 2. The line 22 feeds between the extractive rectifying section 23b and the extractive stripper section 23a of that system, which system serves for the stripping of benzene from the said bottom product, benzene being discharged via a line 24. Propylene carbonate is fed through a line 25 to the uppermost plate of the extractive rectifying section 23b. A fraction containing the non-aromatics is drawn off from the bottom of the distillative rectifying section 23c and passes through a line 26 into the side column 23d, from which the non-aromatics are removed at the bottom and are fed into the manifold line 17 through a line 27.

The bottoms product of the extracting stripper section 23a, which contains residual aromatics in addition to the extraction medium, is pumped through a line 28 into a second column system 29, again arranged as described with reference to FIG. 2, which serves the purpose of stripping the toluene from said bottom product. Propylene carbonate is supplied through a line 30. The extracted toluene is drawn off through a toluene output line 31, some being recycled, as described, to the extractive distillation columns 14 and 19 through the recycle line 32. The side column 29d serves as distillative stripper section and is connected to the bottom of section 29c by a line 33. The non-aromatics are drawn off through a line 34 which extends into the manifold line 17 The bottoms product of the column system 29, containing the xylene as well as the higher boiling aromatics and extraction agent is finally pumped through a line 35 into the final column system 36 which serves for extracting the xylene.

The column system 36 is again arranged as described in relation to FIG. 2 but differs in that, since the yield and purity of the C aromatics to be obtained by extraction of xylene from the bottom product of the column system 29 is immaterial, the system 36 does not include an extractive rectifying section, nor does it have an extraction agent feed.

The xylene is drawn off through a line 37. The side column 36d functions as distillative stripper section and is connected to the bottom of the column section 36c via a line 38. The remaining non-aromatics and the residual aromatics with a higher boiling point than xylene are withdrawn via a line 39 and fed to the manifold line 17. The recovered extraction agent from the bottom of section 36a (predominantly propylene carbonate since toluene has been stripped therefrom by the column system 29) is delivered for reuse via the line 15 and the branch lines 20, 25, 30. Fresh propylene carbonate is supplied through a line 40 when necessary to make good losses and to introduce this constituent of the extraction agent into the process when the plant is started up.

The following yields were obtained with a plant arranged and operated as described above and using the specified feedstock:

Benzene=287 kg.=96.7% of the feedstock benzene. Toluene=115 kg. =76.6% of the feedstock toluene. Xylene=69 kg.=98.6% of the feedstock xylene.

Substantially 100% of the non-aromatics of the feedstock were drawn off through the manifold line 17. The method according to the invention is therefore characterised by good separation and a high yield for all required fractions. All aromatics obtained had a high degree of purity.

In many cases, it may be desirable for the higher boiling aromatics, for example C aromatics, to be extracted in addition to benzene, toluene and xylene. If these aromatics are required to be extracted from the mixture obtained from the bottom of the column section 36d, such mixture may be processed in a further column system. If no quantitative separation of the C aromatics is envisaged, such processing may be accomplished in a simple stripper column.

With respect to the first column system 23, in which the stripping off of benzene is carried out, the following conditions may prevail:

The feed stock, after having been freed in extractivedistillation columns 14 and 19 of a large portion of the non-aromatic constituents, is withdrawn through conduit 22 and introduced into the first column system 23, for stripping off of benzene.

The product introduced into column system 23 has the following composition:

With respect to the toluene content, it has to be considered that in addition to the toluene content of the feed stock (150 kg.) also about 120 kg. toluene had been introduced into the system with the extraction agent.

The benzene which has been stripped off in column system 23, is withdrawn through conduit 24, and has the following composition:

Kg. Benzene 286.8565 Toluene 0.0287 Non-aromatics 0.1148

Thus, the benzene is of a purity of 99.95%.

The non-aromatics which are separated in column system 23, are Withdrawn through conduit 27. This fraction has the following composition:

Non-aromatics 109.032 Benzene 1.943 Toluene 19.932

The aromatics having boiling points higher than benzene accrue in the sump of extractive column 23a and are pumped through conduit 28 into the next following column system 29 in which toluene is stripped off.

This fraction has the following composition:

Kg. Non-aromatics 10.17 Toluene 242.92 Xylene 69.16 C -aromatics 19.76

The now following column systems for the stripping off of toluene and xylene operate in substantially the same manner as described above with respect to the stripping off of benzene.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. Extractive distillation process for simultaneously obtaining different aromatics with different boiling point ranges from a feed stock containing varied amounts of non-aromatics along with the desired aromatics comprising passing the feed stock through at least one extractive distillation column while contacting it in said column with an at least partially non-aromatic selectively active extraction agent containing aromatics other than the aromatics to be recovered; withdrawing a liquid extract of aromatics and extraction agent from the bottom of said column while withdrawing the non-aromatics in vapor form at the top of said column; then passing said liquid extract of aromatics and extraction agent through a plurality of column systems arranged in series, one of said systems being provided for each desired type of aromatic; subjecting said liquid extract of aromatics and extraction agent in each of said systems to an extractive distillation followed by a conventional distillation, and withdrawing the aromatic to be recovered from the respective rectifying distillation, and also withdrawing non-aromatic from said stripping distillation.

2. Process according to claim 1 wherein the mixture of extraction agent, aromatics and non-aromatics in at least one of said column systems is passed in sequence through an extractive stripping section, an extractive rectifying section, a distillative rectifying section and a distillative stripping section.

3. Process according to claim 2 wherein the mixture of extraction agent, aromatics and non-aromatics in at least one of said column systems is passed from an extractive stripping section to a superposed extractive rectifying section and is then passed into a second column formed of two superposed distillation sections.

4. Process according to claim 2 wherein the mixture of extraction agent, aromatics and non-aromatics in at least one of said column systems is passed in succession through a first column composed of three sections comprising an extractive stripping section, an extractive rectifying section and a distillative rectifying section and then into a second column composed of a distillative stripplng section.

5. Process according to claim 1, wherein the amount of aromatics that is added to the extraction agent is up to 20% by volume relative to the volume of the extraction agent.

6. Process according to claim 1 wherein the mixture of extraction agent, aromatics and non-aromatics is passed through at least two successively arranged extractive distillation columns, each successive column being operated with reduced pressure relative to the preceding column.

7. Process according to claim 6 wherein the first of said extractive distillation columns is operated at a pressure above atmospheric.

8. Process according to claim 1 wherein in the last of said columns, said mixture of extraction agent, aromatic and non-aromatic is passed from an extractive stripper section directly to a distillative rectifying section and from there to a distillative stripper section, the desired aromatic fraction being withdrawn from the top of said distillative rectifying section and the refined non-aromatics product being withdrawn from the bottom of the distillative stripper section.

9. Process according to claim 1 wherein in at least one of said systems the mixture of extraction agent, aromatics and non-aromatics is passed from an extractive stripper section to an extractive rectifying section and then to a distillative rectifying section, from which the desired aromatics fraction is withdrawn at the top.

10. Process according to claim 2 wherein the extraction agent that has been stripped from the aromatics to be recovered is withdrawn from the last extractive stripper section and is fed back into the distillation process.

11. Process according to claim 7 wherein the sensible heat of the bottoms product of the last extractive stripper column and of the high-pressure operated extractive distillation column is employed for heating the lowerpressure extractive distillation columns and the other stripper columns.

12. Process according to claim 1 wherein a portion of the aromatics used in the extraction agent is introduced into the extractive distillation columns separate from the extraction agents at a place above the inlet for the remainder of the extraction agent.

13. An extractive distillation apparatus, comprising at least two extractive distillation columns arranged in series and having connected to the last extractive distillation column a plurality of column systems arranged in series, at least one of said column systems being comprised of an extractive stripper section, an extractive rectifying section superposed on said extractive stripper section, and a distillative rectifying section superposed on said extractive rectifying section, and, connected with said distillative rectifying section, a separate distillative stripper section, means for introducing the mixture of extraction agent, aromatics and non-aromatics obtained from said extractive distillation columns at a point in proximity to the borderline between the extractive stripper section and the extractive rectifier section of each column system, outlet means at the top of said distillative rectifying section for the desired aromatics, conduit means for passing the extraction agent and non-aromatics from the bottom of 3 the extractive stripper section to the next column system and conduit means for feeding back the extraction agent from the last extractive stripper section to the first ext stt t st a e sa sa-ea 14. An extractive distillation apparatus, comprising at least two extractive distillation columns arranged in series and having connected to the last extractive distillation column a plurality of column systems arranged in series, at least one of said column systems being comprised of an extractive rectifying section superposed on said extractive stripper section, and a distillative rectifying section and a distillative stripper section, the last two sections being superposed on each other, means connecting the top of said extraction rectifying section and a point in proximity to the boundary between said two distillative sections, outlet means at the top of said distillative rectifying sections for the desired aromatics, conduit means for passing the extraction agent and non-aromatics from the bottom of the extractive stripper section to the next column system, and conduit means for feeding back the extraction agent from the last extractive stripper section to the first extractive distillation column.

References Cited UNITED STATES PATENTS 2,325,379 7/1943 Durrum 203-60 2,350,256 5/1944 Shiras et al 203-84 2,357,028 8/1944 Shiras et al. 203-84 2,455,803 12/ 1948 Pierotti 203-84 2,460,852 2/ 1949 Shiras et al 203-84 2,537,459 l/1951 Griswold 203-84 2,961,383 11/1960 Black 203-84 2,981,663 4/1961 Black 203-82 3,058,893 10/1962 Cahan et al 203-82 3,445,537 5/ 1969 Luther et al. 260-674 FOREIGN PATENTS 793,151 4/ 1958 Great Britain 260-674 WILBUR I. BASCOMB, 1a., Primary Examiner US. Cl. X.R. 

